Alternating current bridges (rectifiers) are used for rectification in three-phase or alternating current generators (alternators) of motor vehicles. Semiconductor diodes having a p-n junction made of silicon are used in most cases as rectifying elements. Six semiconductor diodes are, for example, interconnected in a three-phase generator to form a B6 bridge. Occasionally, diodes are also connected in parallel, for example, twelve diodes are used instead of six diodes. In alternating current generators having a different number of phases, accordingly adapted diode bridges are used. The diodes are configured for operation at high currents having current densities of more than 500 A/cm2 and at high temperatures having a maximum junction temperature Tj of approximately 225° C. The voltage drop in the forward direction, forward voltage UF, is typically approximately 1 volt for the high currents used. In the case of operation in the reverse direction, only a very low reverse current IR up to a breakdown voltage UZ generally flows. Starting from this voltage, the reverse current increases very strongly. Another voltage rise is thus prevented.
In most cases, Zener diodes (Z diodes) having reverse voltages of approximately 20 volts to 40 volts are used in this context, depending on the electrical system voltage of the motor vehicle. At breakdown, Z diodes may briefly be subjected even to very high currents. They are therefore used for delimiting the overshooting generator voltage in the event of load changes (load dumps). Such diodes are usually packaged in robust press-in diode housings, as described in German Application No. DE 195 49 202 B4, for example.
One disadvantage of such a device is that the forward voltage of the p-n diodes results in conducting-state power losses and thus in an efficiency deterioration of the generator. Since two diodes are on average connected in series during a power output of the generator, the averaged conducting-state power losses in a 100-A generator are approximately 200 W. The associated heating of diodes and rectifiers must be reduced by complex cooling measures, e.g., by using heat sinks or fans.
German Application No. DE 10 2004 056 663 A1 proposes to use so-called high-efficiency diodes (HEDs) instead of the p-n diodes to reduce the conducting-state power losses. High-efficiency diodes (HEDs) are Schottky diodes which, in contrast to conventional Schottky diodes, do not have the barrier lowering effect (BL) caused by the reverse voltage and thus have low reverse currents. High-efficiency Schottky diodes (HEDs) include a combination—monolithically integrated on a semiconductor chip—of a conventional Schottky barrier diode (SBD) together with other elements such as magnetoresistors, p-n junctions, or different barrier metals. They are often implemented in trench technology.
With the aid of high-efficiency Schottky diodes (HED), essentially lower forward voltages UF may be implemented which are in the range of 0.5 V to 0.6 V. The low conducting-state power losses of the diodes increase the efficiency and the output power of the particular generator. Since Schottky diodes, as majority carrier components, switch very quickly, the radio interference suppression of the generator additionally improves in certain frequency ranges by up to 10 dB.
Due to the lower reverse power losses, the complexity for cooling the diodes may be additionally reduced compared to the use of p-n diodes.
A production of high-efficiency Schottky diodes (HEDs) is, however, complex and technically very sophisticated. In addition to the very fine trench structures, which have mesa widths in the range below 500 nm and must be etched into the silicon, a cost-effective production of suitable and stable Schottky contacts represents a challenge, in particular. Nickel silicides or other suitable silicides are preferably used as Schottky contacts. In modern semiconductor plants, in which power MOSFETs are produced, these silicide processes are usually not available.